In common optical disc drive systems like CD-ROM drives and digital versatile disc (DVD) players, a flatbed motor is used to drive a flatbed having an optical read head for performing tracking and seeking actions for an optical disc.
A plurality of tracks for recording data is located on an optical disc. The so-called seeking action moves the optical read head to a track having data to be read. The seeking action can be divided into short seeking and long seeking. Short seeking generally means under 1000 tracks are searched. Short seeking is necessarily quick and accurate. Therefore, a closed loop control is required. On the other hand, quick seeking is required for long seeking. Therefore, an open loop control is required. In order to keep the object lens in the central position, a central error control is performed on a tracking actuator. Usually, an accurate short seeking is performed after a long seeking for positioning.
The tracking action is a horizontal motion of a lens for locking onto the track to be read. After the tracking action, a laser beam illuminates the optical disc. The reflected light is received by a photodetector on the optical read head. Original signals required for data signals on the optical disc and various controls are then output.
Signals obtained by the optical read head are combined by a front-stage amplifier into a radio frequency (RF) signal and some control signals such as a tracking error (TE) signal, a radio frequency ripple (RFRP) signal, a tracking error zero cross (TEZC) signal and a radio frequency zero cross (RFZC) signal. Existent optical disc drives make use of the RFZC signal and the TEZC signal to generate the counting track mechanism for short seekings.
The RFRP signal is obtained from the read RF signal. The RF signal is a data signal read from the optical disc. When the lens is aligned with a track, the RF signal is at maximum amplitude. When the lens is between two tracks, the RF signal is at minimum amplitude. The RFRP signal is obtained by subtracting the lower envelope from the upper envelope of the RF signal, or performing a low-pass filtering on the RF signal.
The RFZC signal is obtained from the read RFRP signal. In a conventional method, a fixed value is set as a slice level when performing tracking actions. For instance, the zero value of the amplitude of the RFRP signal is set as the slice level. If the value of the RFRP signal is greater than the slice level, the value of the RFZC signal is high. If the value of the RFRP signal is less than the slice level, the value of the RFZC signal is low. The main function of the CD-ROM drive's RFZC signal is to count tracks, and can be used regardless of long or short seeking control.
In another conventional method, the slice level is generated by a hardware circuit low-pass filter. As shown in FIG. 1, a conventional hardware low-pass central level generator comprises a capacitor (C) 100, a resistor (R) 102 and a comparator 104. The RFRP signal is input via terminal X. A reference voltage (Vref) is input via terminal Y. The RFZC signal is output via terminal Z. The RFRP signal passes through the low-pass filter composed of the capacitor 100 and the resistor 102 and is then compared with the RFRP signal by the comparator 104 to generate the RFZC signal.
However, when regions with data and regions without data of an optical disc are staggered, the RFRP signal obtained from the regions with data has a greater amplitude while the RFRP signal obtained from the regions without data has a lesser amplitude. If the above conventional methods are adopted, the obtained RFZC signal is distorted to cause problems in seeking actions.